Any stage or point in an audio/video signal path where the gain or level of the signal can be adjusted or amplified. Basically this means any piece of equipment the signal passes through can be considered at least one gain stage (if not many stages). Any place your signal encounters a level control is a gain stage. That means the “input” AND “output” level controls of something are both gain stages. It is desirable to have gain stages matched throughout the path of a signal. Boosting a signal way up at one point while turning it way back down at the next point is generally not preferred as it can reduce the overall dynamic range and signal to noise level available.